Seals employing a magnetic fluid have been used and are known to provide a positive hermetic barrier against gas and other contaminants independent of differential pressure or direction. Such magnetic fluid seals are further characterized by their long life, high reliability, low or immeasurable discharge and cost effectiveness in many rotary shaft systems. Pioneering work in the field of magnetic fluid seal apparatus and particularly multi-stage seals was disclosed in U.S. Pat. No. 3,620,584, issued on Nov. 16, 1971, hereby incorporated by reference. Additionally, U.S. Pat. No. 3,917,538, issued on Nov. 4, 1975, also incorporated by reference, revealed novel compositions and methods of making magnetic fluids, also known as ferrofluids, having excellent magnetic characteristics. Ferrofluid seals operate in principle through the use of ferrofluid in a gap between rotary and stationary surfaces in the presence of a magnet to provide focused radial magnetic flux lines which retain the ferrofluid, for example, in a liquid "O-ring" in the gap about a rotating shaft.
Magnetic fluid exclusion seals have recently found usage in computer magnetic-disc-storage units to prevent contaminants from reaching the disc area. Computer disc drives typically employ a magnetic fluid-type exclusion seal module encased in a stationary nonmagnetic housing to prevent the ingression of airborne or other particulates and vapors up and around the disc-drive shaft.
In computer disc-drive applications, the seal module comprises: a permanent ring magnet polarized axially, a pair of magnetically permeable pole elements which sandwich the magnet and a magnetic fluid. Typically the seal is magnetically isolated from the disc memory to prevent the seal low-strength magnetic field from interfering with the information stored on the disc. The seal is assembled about the rotary disc shaft so that the inner peripheral ends of the pole pieces form a gap with the shaft surface; i.e., 4-8 mils. A magnetic fluid is then painted or otherwise applied in the gaps to form a liquid "O-ring" seal which completes the magnetic flux circuit with the magnetically permeable disc shaft.
Presently, in most computer disc-drive applications as well as other applications the housing with magnet and pole pieces encased therein is purchased as a unit; however, the user must purchase and apply the magnetic fluid at the time of installing the seal about the shaft. Application of the magnetic fluid by the user presents a number of difficulties. The fluid is expensive and sometimes difficult to obtain in small quantities. As with other fluids, leakage during storage must be avoided. Additionally, application of the magnetic fluid requires a certain amount of skill and it is difficult to determine when the correct amount of magnetic fluid has been applied. Thus, it would be most desirable for magnetic fluid exclusion seals to be self activating or to avoid problems associated with the addition of ferrofluid at the point of assembly.